1. Field of the Invention
The present invention relates to an image forming apparatus such as a digital color copying machine.
2. Description of the Prior Art
In a digital color copying machine, a document is read to obtain digital data of red, green and blue. The digital data are converted to image data of cyan, magenta, yellow and black to be reproduced on a sheet of paper. The digital data are subjected to various processing such as shading correction, density conversion, color correction, edge emphasis, smoothing, gamma correction and the like.
Prescan of the document is performed for determining the conditions on forming an image of the digital data in a sheet of paper. Data obtained with a prescan on a document to be reproduced is performed for detection of document size, shading correction or the like. For example, automatic exposure is performed on prescan data for correcting image data to reproduce background of a document as white. However, the automatic exposure processing is effective only for a monochromatic character image because a reproduced image becomes dull for a color image or a photograph image. It is desirable that automatic exposure processing is performed for a color image or the like.
Auto color selection is also performed on prescan data to determine if a document is a monochromatic image or a full color image. Image data on the document is processed according to the auto color selection.
If the automatic exposure processing and the auto color selection are performed at the same time, a document size has to be detected beforehand. Then, prescan data are used twice. However, it is not desirable to perform prescan twice because it takes a longer time for copying.
A digital copying machine reads a document to get digital data of red, green and blue thereof and converts them to image data of cyan, magenta, yellow and black, which are reproduced on a sheet of paper with toners having the four colors. The image data are subjected to MTF correction before printing. For a document image comprising black characters, it is desirable to emphasize edges of black characters. Then, for example, black characters are detected in a document image, and the image data for the black characters are subjected to edge emphasis and are reproduced with black toners. Thus, the black characters are reproduced with black toners, and the image quality is improved.
As to the black characters, image data of the cyan, magenta and yellow are suppressed at the edges according to brightness component of the image data. However, if black characters are described in a colored background, an amount of suppression of the image data of the cyan, magenta and yellow becomes large, so that areas adjacent to the black characters become white though they have a color of the colored background. This deteriorates image quality in a reproduced image.
Black characters are detected as black image in an area between a pair of edges. However, for characters of dark blue, dark green or the like, edges are liable to be decided erroneously. Further, for an image comprising black dots, edges are decided erroneously, a Moire pattern may happen. It is also a problem that in an electrophotographic copying machine, toners are liable to have a higher density at a leading edge and a lower density at a trailing edge, and this also deteriorates image quality of black characters.
A digital copying machine can control image qualities on various items such as hue, chroma, color, balance and the like besides gamma correction and edge emphasis. When a user wants a copy of a desired image quality, he or she adjusts the various items before starting copying operation. If the reproduced image does not have the desired image quality, he or she has to adjust the various items again based on the reproduced image. However, because a number of items to be adjusted is large, it is difficult to predict an image quality. Then, it is desirable to adjust image quality efficiently.
As to color balance, density level of toners of cyan, magenta, yellow and blue is controlled independently of each other. Then, density on a sheet of paper is also affected by adjustment of the color balance. Then, a total image density of a reproduced image may become higher or lower, and an amount of toners fixed on paper becomes uneven. It is also a problem that jam or the like may happen.
A first object of the present invention is to provide an image processor which performs prescan efficiently.
A second object of the present invention is to provide an image processor which performs automatic exposure appropriately irrespective of kind of document.
A third object of the present invention is to provide an image processor which decides black characters more precisely for edge emphasis.
A fourth object of the present invention is to provide an image forming apparatus which is easy to be adjusted on image quality.
A fifth object of the present invention is to provide an image forming apparatus which can adjust color balance appropriately.
In an aspect of the invention, color data of a document is read by a scanner and the color data are converted to lightness data and chroma data, and a black edge is detected by deciding an edge from the lightness data and deciding a black color from the chroma data. Then, the data of black of an interest pixel is increased by a prescribed amount for edge emphasis if the pixel is decided to exist at a black edge. Preferably, the color data are converted to data of cyan, magenta, yellow and black, and the data of cyan, magenta and yellow of a pixel at a black edge are replaced with minima of data of cyan, magenta and yellow of pixels in a prescribed region around the pixel. Preferably, for an isolated pixel, an amount to be added for edge emphasis is limited according to the density of the isolated pixels.
In a second aspect of the invention, in order to prevent erroneous decision of a black edge, each of the pixels in a prescribed area around an interest pixel to be decided to exist at a black edge are checked if it has a color based on the lightness data and chroma data and is not located at the edge. If the number of the pixels having colors and not existing at the edge is larger than a threshold level, the decision as a pixel at a black edge is invalidated.
In a third aspect of the invention, a black edge is detected by deciding an edge from the lightness data and by deciding a black color from the chroma data. On the other hand, area information on a document is received in synchronization with read of the document, and a type of the document is determined. Then, the data of the interest pixel are changed according to the type of the document and a result of decision of black edge.
In a fourth aspect of the invention, a color document is read, and a pixel at a rising edge and a pixel at a trailing edge of an image in the color data with respect to a paper-feed direction. Then, on edge emphasis, a first correction data is added to a pixel located by one pixel before the rising edge, and a second correction data is added to a pixel located one pixel after the trailing edge.
In a fifth aspect of the invention, automatic exposure is performed on a color document for correcting a background of the document. Color data of a color document are read, while a background of the document is obtained as a background level detected from a density histogram of pixels decided to be black. That is, the background level is determined only on pixels decided to be black. Then, the color data are corrected according to the background level for automatic exposure. Alternately, a standard document such as a white plate is also read, and the data of the standard document is subjected to the shading correction. A background level is also determined only on pixels decided to be black. Then, the color data of the document are corrected according to a difference of the background level or the document from the background level detected on the standard document so that a background level of an image formed on the sheet of paper has a prescribed value. Preferably, when color data of a document is read, an area wherein the document exists on a platen is determined. Then, an underground level of the document and a type of the document such as full color document are determined on the same data in the area.
In a sixth embodiment of the invention, a part of a color document read by a scanner is formed in a sheet of paper with different levels of an image forming condition such as edge emphasis. That is, image data on the same part of the color data are supplied repeatedly so that the plurality of image data is formed on a sheet of paper. A data processing means processes the part of the color data by changing the image forming condition successively. Then, a plurality of color image of the part processed on the image forming condition is formed in a sheet of paper.
In a seventh aspect of the invention, a scanner reads a color document to provide color data, and the color data are converted to data of cyan, magenta, yellow and black. When color balance is adjusted on one of the data of cyan, magenta, yellow and black, the data for each pixel of cyan, magenta, yellow and black are changed according to the color balance while keeping a total of the data of cyan, magenta, yellow and black constant. That is, when color balance is adjusted, the density is kept constant.
An advantage of the invention is that edge emphasis is performed appropriately.
Another advantage of the invention is that erroneous decision of black edge can be prevented for example on dark blue characters.
A third advantage of the invention is that an underground level of a color document can be corrected appropriately.
A fourth advantage of the invention is that a document area, an underground level and the like can be corrected in a single read of a document data.
A fourth advantage of the invention is that a user can select desired image forming conditions easily.
A fifth advantage of the invention is that color balance can be performed while keeping a constant density.